This invention relates to a heat-insulating paper container having on its wall surface a foamed layer of a thermoplastic film and a method for producing such a container.
Several types of heat-insulating containers have been used commercially to pack hot liquids. A polystyrene foam heat-insulating container is one example. It is prepared by casting unfoamed polystyrene into a mold, heating the resin under pressure to foam it, and removing the foamed resin from the mold. Alternatively, a foamed styrene sheet may be shaped into a container. The resulting container has fairly high heat insulating ability, but it is not competitive costwise. With the current oil price hike and expected depletion of oil resources that demand an economical method for manufacturing heat-insulating containers with less energy, the high cost of the conventional method is a serious defect. As a further disadvantage, the outer surface of the foamed styrene heat-insulating container is not smooth and has low printability.
The conventional paper heat-insulating container can not be manufactured at low cost, and one reason is the complexity of the manufacturing process. One example is a container wherein the side wall of the body member is surrounded by a corrugated heat-insulating jacket. The process of manufacturing such container involves additional steps of forming the corrugated jacket and bonding it to the outer surface of the side wall of the body member. One defect of this type of container is that letters, figures or other symbols are printed on the corrugated surface and the resulting deformed letters or patterns do not have aesthetic appeal to consumers. Another defect is that the jacket is bonded to the side wall of the body member in such a manner that only the valley ridges contact the side wall, and the bond between the jacket and the side wall is so weak that the two will easily separate. Another type of paper heat-insulating container has a "dual" structure wherein an inner cup is given a different taper than an outer cup to form a heat-insulating air layer. The two cups are made integral by curling their respective upper portions into a rim. The side wall of the outer cap is flat and has high printability but again, the two cups easily separate. Another defect is that the dual structure increases the manufacturing cost. Therefore, the development of a paper container having great heat-insulating ability and which can be manufactured easily at low cost has been desired.